mr.m13245 told you how to make a video with Movie Maker, but, gave you some bad advice about limewire. Using almost any music that you find online is going to be a copyright violation and will put your account at risk. But, yes you can use a song from YouTube if you mean using their audioswap function. To do that, you would make the video in Movie Maker and after you post it go to edit on your video and select audioswap from the list pick something that is about the same length as your video.

>youtube in your videos allows you to download an mp4 file. So this would include your new Audio and Video, and thus download back to your PC.

Nope. Once you have used audioswap you can no longed download the mp4 version, to allow that would violate copyrights of the music owners who have only licensed it to be used on YouTube.

>Hope this helps!! (PS: Limewire is technically illegal file sharing)

That may be true, and the point is? Getting the music legally does not invalidate the fact that to then use that music in ones video is going to be a copyright violation. So, get it from limewire or from iTunes or off a CD, but, to do so will put your video and channel in peril.